The present invention relates to non-blocked, nanoparticle-modified polyisocyanates which are distinguished by increased stability in their NCO content.
A series of patents describe surface-functionalized particles having groups that are potentially reactive towards the film-forming resins, and their use in coatings (EP-A 0 872 500, WO 2006/018144, DE-A 10 2005 034348, DE-A 199 33 098, DE 102 47 359). The particles in question include nanoparticles which carry blocked isocyanate groups, and dispersions thereof, which are used in a blend with binders. These systems are stable even on prolonged storage, in terms of their NCO content, which suffers no degradation.
EP-A 0 872 500 and WO 2006/018144 disclose, for example, colloidal metal oxides whose nanoparticle surfaces have been modified via covalent attachment of alkoxysilanes. The alkoxysilanes used for the modification are addition products of aminoalkoxysilanes and blocked, monomeric isocyanates. Metal oxides modified in this way are then mixed with the binders and curing agents and used as an isocyanate component for the production of coating materials. Essential to the invention here is the presence of water and alcohol in the preparation process for the hydrolysis of the alkoxy groups, with subsequent condensation on the particle surfaces, producing a covalent attachment. Likewise essential to the invention is the blocking of free NCO groups in order to prevent reaction with water and alcoholic solvent. The systems in question here, therefore, are modified nanoparticles and not nanoparticle-containing polyisocyanates. On reaction, accordingly, the nanoparticles are incorporated covalently into the film-forming matrix and hence dominate the film-forming matrix, which from experience can lead to detractions in terms of the flexibility. It is disadvantageous, moreover, that owing to this process, which makes the use of water and alcoholic solvent essential, it is not possible to use non-blocked polyisocyanates.
Farbe and Lack 7 (2007), p. 40 ff reports the coagulation of nanoscale silica particles as a result of addition of polyisocyanates to a two-component coating formulation. The silica particles are located in the polyol phase; the polyisocyanate is added as second component. As a result of addition of carboxylic acids to the mixture of polyol and silica particles, coagulation after blending with polyisocyanate is suppressed. There is no description of corresponding formulations, in the form of storage-stable products in polyisocyanate, in which the isocyanate content is not reduced over time. Carboxylic acids can react by reaction with polyisocyanate and lead to unwanted secondary reactions, and hence adversely affect the film-forming properties.
Applications DE 10 2006 054289 and EP 07021690.2 disclose colloidally stable, transparent and translucent, respectively, nanoparticle-containing polyisocyanates which are obtained by modification of polyisocyanates with aminoalkoxysilanes and aminoalkoxysilanes respectively and polydimethylsiloxanes and addition of nanoparticles. On storage, however, nanoparticle-containing polyisocyanates of this kind are unstable in terms of their NCO content, as a function of the solvent used.
Starting out from the prior art, then, the problem addressed was that of providing non-blocked, nanoparticle-containing polyisocyanates, especially for 2K (2-component) polyurethane applications.
It has now surprisingly been found that such polyisocyanates can be obtained by preceding contact between polyisocyanate to be modified and nanoparticles by a special surface modification of the particles.